Processes that treat mineral ores in order to extract mineral values including in some cases the recovery of hydrocarbons will normally result in waste material. Often the waste material consists of an aqueous slurry or sludge comprising particulate mineral material, for instance clay, shale, sand, grit, metal oxides, etc. admixed with water and possibly residual hydrocarbons.
In some cases the waste material such as mine tailings can be conveniently disposed of in an underground mine to form backfill. For other applications it may not be possible to dispose of the waste in a mine. In these instances, it is common practice to dispose of this material by pumping the aqueous slurry to open mines, pits, lagoons, heaps or stacks and allowing it to dewater gradually through the actions of sedimentation, drainage, evaporation, and consolidation.
There is a great deal of environmental pressure to minimize the allocation of new land for disposal purposes and to more effectively use the existing waste areas. Generally, the goal of oil sands tailings remediation is to recover water to be reused in the process and obtain solids that are dry enough so as to be trafficable so that the dried tailings/solids can be used to reclaim land. The exact requirements needed for final land reclamation and restoration will often be governed by regional or national mandates.
One method is to load multiple layers of waste onto an area to thus form higher stacks of waste. However, this presents a difficulty of ensuring that the waste material can only flow over the surface of previously consolidated waste within acceptable boundaries, is allowed to rigidify to form a stack, and that the waste is sufficiently consolidated to support multiple layers of consolidated material, without the risk of collapse or slip. Thus the requirements for providing a waste material with the right sort of characteristics for stacking is altogether different from those required for other forms of disposal, such as back-filling within a relatively enclosed area.
In a typical mineral processing operation, waste solids are separated from materials that contain mineral values in an aqueous process. The mined material may also encompass the recovery of hydrocarbons. The aqueous suspension of waste solids often contains clays and other minerals, and is usually referred to as tailings. This is true in a variety of mineral solids including tailings from oil sands. These solids can be concentrated by a flocculation process in a thickener to give a higher density underflow and to recover some of the process water. It is usual to pump the underflow to a surface holding area, often referred to as a tailings pit or dam. Other ways to treat the flocculated solids include, for example, thin-lift which involves depositing on a slope with dewatering and evaporation, deep-pit, filter presses, belt presses, or centrifuges.
Once deposited at this surface holding area, water will continue to be released from the aqueous suspension resulting in further concentration of the solids over a period of time. Once a sufficient volume of water has been collected this is usually pumped back to the mineral processing plant. Once the solids content is high enough in the dewatered solids, land reclamation becomes possible.
Sometimes the solids content is not high enough after a first treatment and tailings may need to be retreated to further increase its solid content.
Flocculants are also used to treat legacy tailings, sometimes referred to as mature fine tailings (MFT), such as those collected and concentrated in ponds.
The prior art contains numerous references to flocculation processes and various proposed flocculating agents, for example see U.S. Pat. No. 7,901,583.
Polymeric flocculants include polymers of sodium acrylate, for example see U.S. Pat. No. 4,342,653 and sulfonate polymers, for example see U.S. Pat. No. 4,704,209.
Polymers of acrylamide are commonly employed as flocculating agents. However, acrylamides suffer from various disadvantages, for instance, rapid hydrolyzation in alkaline solutions which makes them impractical for use in various applications that involve alkaline solutions, sensitivity to treatment dosage levels which makes it easy to under or overdose, floc shear instability, and/or floc structures which do not allow for complete dewatering.
U.S. Pat. No. 4,678,585 describes flocculating red mud suspensions in the Bayer alumina process using homopolymers of acrylic acid or sodium acrylate or copolymers of acrylic acid or acrylate with acrylamide.
Li, et al., Energy & Fuels, 2005, vol. 19, pp. 936-943 disclose the effect of a hydrolyzed polyacrylamide (HPAM) on bitumen extraction and tailings treatment of oil sands ores. Careful control of HPAM dosage is necessary to achieve efficiency in both bitumen extraction and in flocculation of solid fines.
Polymers of polyethylene oxide are commonly employed as flocculating agents, for example see U.S. Pat. Nos. 4,931,190; 5,104,551; 6,383,282; and WO 2011070218. A primary disadvantage of such use of polymers of polyethylene oxide is that they produce small floc structures. US Publication No. 20120160776 discloses stimulant-sensitive flocculation using a polyethylene oxide/polypropylene block copolymer.
In spite of the numerous polymeric flocculating agents, there is still a need for a flocculating agent to further improve consolidation of suspensions of materials as well as further improve upon the dewatering of suspensions of waste solids that have been transferred as a fluid or slurry to a settling area for disposal. In particular, it would be desirable to provide a more effective treatment of waste suspensions, such as oil sands tailings, transferred to disposal areas ensuring fast, efficient concentration, and more environmentally friendly storage of solids and improved clarity of released water.